C++ definitions are numerous, and they shouldn't pollute variable space and
performance outside of directories containing C++ files. This commit pushes
them into a defs-cpp.mk file.
Signed-off-by: Jan Lindemann <jan@janware.com>
Add support for GNUmakefile, where originally only Makefile was supported.
Not done everywhere, as some of the code involving makefiles is dead
anyway.
Signed-off-by: Jan Lindemann <jan@janware.com>